The present invention relates generally to the field of nuclear reactors and more particularly to an apparatus and method for inspecting spring hold down bolts of an upper tie plate of a fuel rod assembly within a nuclear reactor.
Bolt failures in nuclear reactors are frequently a consequence of irradiation. Inspection of bolts in the core area and in the spent fuel pool is therefore important.
Nuclear reactors of well known design typically utilize assemblies of fuel rods which are arranged with their axes parallel to each other in a generally square or rectangular matrix. Appropriate spacers are employed to arrange the fuel rods in the desired matrix. An element known as a tie plate is commonly used at least at one end of the fuel rod assembly in order to facilitate movement of the assembly and to constrain the rods. The fuel rod assemblies are supported within the reactor in a generally vertical orientation and the tie plate located at the upper end of the vertical arrangement, known as the upper tie plate, is used for proper positioning of the fuel rods in the desired matrix. Common designs for the upper tie plate incorporate leaf springs, usually positioned along the horizontal edges of the tie plate, in order to provide proper tensioning of the tie plate against other elements in the reactor. Corner clamps are used at opposing corners of the tie plate to hold down at least one end of the leaf springs. The corner clamps are angle assemblies secured to the tie plate by a pair of hold down bolts. Other corners of the tie plate are usually provided with guide pin holes for receiving guide pins to properly align the tie plate with other assembly elements in the reactor.
It has been found that the leaf spring hold down bolts sometime develop flaws, such as fine cracks, when subjected to reactor cycling. When such flaws develop, the hold down bolts may fail to adequately hold the leaf springs in proper position. The springs that are used to provide tensioning forces therefore no longer function properly. This can result in certain parts, such as springs, clamps, and/or bolts, of the upper tie plate becoming loose. If the bolts break, the leaf springs fail to function properly and the fuel rod assembly can move vertically, which is not desirable. In addition, appropriate fuel assembly handling equipment might not be able to properly grip the assembly for desired movement or placement. When the bolts fail, the corner clamps that position and secure the springs become free. This allow the springs to move freely (still constrained by the upper tie plate). When the fuel assembly is to be moved, the fuel handling grapple must interface with the upper tie plate alignment hole and have clearance to engage with the inside of the upper tie plate lip. In an assembly with broken bolts, the springs typically block access to the inside lip of the upper tie plate, making engagement and grappling impractical. Under extreme cases, the clamp block may become free and potentially damage reactor internals. Fuel assemblies with broken bolts cannot be returned to the core until repaired.
Even without knowing whether certain bolts indeed have flaws that could lead to serious defects, replacement of the upper tie plates is nonetheless undertaken at great cost and involve time consuming procedures. No effective means for directly inspecting the bolts to determine the pressure of incipient flaws without having to remove them is currently in use.
It is accordingly a general object of the present invention to provide a method and apparatus for effective and rapid inspection of the spring hold down bolts on the upper tie plate of the fuel rod assembly in order to overcome the possible needless, but expensive and time consuming, replacement of all upper tie plate assemblies when such replacement might not be necessary.
It is a more specific object of the present invention to provide an apparatus and method for quick, easy and inexpensive inspection of the hold down bolts of the corner clamps for the upper tie plate while the bolts remain in place.
It is a still further specific object of the present invention to provide an apparatus for conducting such inspection which includes a fixture that is adapted to be aligned with the upper tie plate and placed into an inspection position which is located proximate to the tie plate for conducting effective inspection of the hold down bolts insitu.
Yet a further object of the present invention is to provide a means to avoid costly and unnecessary replacement of the upper tie plates which have certain bolts with no flaws without disassembling the hold down assembly arrangement, thus saving significant time and money when such disassembly is not necessary.
A further more specific object of the present invention is to provide a method and apparatus for conducting such inspection utilizing ultrasonic transducers carried by probes on a fixture which can be placed in proximity to the upper tie plate so that ultrasonic scanning of each bolt can be rapidly and inexpensively conducted to determine whether or not any of the bolts have early indications of flaws.
Yet another object of the present invention is to utilize spherically focused ultrasonic transducers which are provided with lenses that cause the focus of the sound field of the transducer at a prescribed distance, which can be located at a position at the bottom of the bolt head so that the sound field beyond the focal point is capable of encompassing the bolt shank above the bolt thread relief. Providing transducers of this type and containing the sound field in this area of the bolt greatly increases test sensitivity.
Another object of the present invention is to provide an apparatus and method for inspecting hold down bolts by placing a fixture on the tie plate assembly and simply viewing ultrasonic data from scanning each bolt.
A further object of the present invention is to provide a rapid and effective means for detecting flaws, such as cracks, having a depth as small as one tenth of one inch.
Other objects, features and advantages of the present invention will become apparent from the detailed description hereinafter.
The invention is directed to an apparatus and method for inspecting spring hold down bolts of an upper tie plate of a fuel rod assembly in a nuclear reactor and includes an inspection fixture adapted to be aligned with the upper tie plate of the nuclear reactor fuel rod assembly for placement into a position for rapid and easy insitu inspection of the hold down bolts of the upper tie plate. The fixture is provided with a plurality of ultrasonic transducers which are located on the fixture so that when the fixture is placed into the inspection position each transducer will be positioned proximate to one of the hold down bolts for conducting effective ultrasonic scanning of the bolt while the bolt is in place. A multi-channel ultrasonic flaw detector is connected to each of the transducers for indicating whether there are any flaws in each of the hold down bolts when the fixture is placed into the inspection position and the ultrasonic transducers are activated. A further feature of the invention is the use of spherically focused transducers for focusing the sound field at a predetermined distance from the head of the bolt in order to facilitate proper scanning of the bolt shank and thread relief, thereby greatly increasing inspection sensitivity.
The foregoing and other features of the present invention are more fully described with reference to the following drawings annexed hereto.